An arc furnace is an electrically operated furnace used for melting metal and/or for cleaning slag. The operation of the furnace is based on a light arc that burns either between separate electrodes, or between electrodes and the material to be melted. The furnace may be operated either by AC or DC current. Heat is created in the light arc, and also in the material to be melted, in case the light arc burns between the material and the electrodes. Power is conducted to vertical electrodes that are located symmetrically in a triangle with respect to the midpoint of the furnace. The assembly depth of the electrodes in the furnace is continuously adjusted, because they are worn at the tips owing to the light arc.
The electrodes extend into the furnace via through holes located in the furnace ceiling. The diameter of a through hole is larger than the diameter of an electrode, in order to ensure free motion of the electrode, and in order to avoid contact between the electrode and the ceiling. The gap left between the electrode and the ceiling aperture must be sealed in order to prevent the access of gases from inside the furnace through the aperture to the atmosphere, and on the other hand in order to prevent the access of air from the atmosphere to the furnace.
In the prior art there are known sealing devices for sealing the gap left between the electrode and the ceiling aperture by mechanical sealings, for instance by graphite rings, braided rope seals etc. that are hydraulically pressed against the electrode. Various mechanical sealing arrangements are known for example from the publications FI 81197, FI 64458, DE 1540876, and SE 445744. The hydraulic medium used for creating hydraulic compression is water.
A drawback with mechanical sealing devices arises in that in practice, the electrode surface is not perfectly cylindrical and smooth, but it may be out-of-round and uneven, which results in the wearing of the sealings that are in contact with the external surface of the electrode as the electrode moves vertically. Thus the sealing is weakened. In arc furnaces with a reducing atmosphere, any leakage of air into the furnace cannot, however, be allowed. On the other hand, a carbon monoxide atmosphere prevails inside the furnace. Again, any leakage of carbon monoxide to the exterior of the furnace cannot be allowed, because carbon monoxide is very toxic. Further, if air flows into the furnace, the carbon monoxide begins to burn and rises the temperature at the aperture very high, thus destroying the furnace structures. The element of a Söderberg electrode that is located inside the furnace is incandescent graphite. Leakage air causes burning and rapid wearing of the graphite, which increases the consumption of both the Söderberg electrode paste and coke.
Another drawback is the use of water in connection with sealing, because in a damage situation water may accidentally get into the furnace. When water is introduced into the furnace atmosphere with a high temperature, a dangerous water-gas explosion may occur.